[81/n] Skip Rechability Transitions in Observer

When the user double-taps a Change of type
TRANSIT_MOVE_LETTERBOX_REACHABILITY is created.

The transition is handled in LetterboxAnimationHandler
where the Letterbox Surfaces bounds are animated.
Nothing triggered by DelegateLetterboxTransitionObserver
should be done in addition to that.

Flag: EXEMPT Refactoring
Fix: 431893696
Bug: 430486865
Test: atest WMShellUnitTests:DelegateLetterboxTransitionObserverTest

Change-Id: Ibc0a3f05f1e1d8de3ca1bca89574945fcea852e4